For a structure of an artificial satellite, a honeycomb sandwich structure formed of skin materials made of fiber reinforced plastic and a honeycomb core, which is lightweight with high stiffness, is generally used. In particular, the honeycomb sandwich structure having high stiffness is used in a mission mounted structure.
However, thermal deformation occurs in the honeycomb sandwich structure owing to a change in thermal environment on an orbit resulting from, for example, input of sunlight heat and heat generation of mounted equipment. Therefore, an angle of an Earth-oriented axis in mission equipment, such as a camera and an antenna, mounted on an artificial satellite varies. In particular, in a stationary satellite located about 36,000 kilometers away from the Earth, even a slight variation in the angle of the oriented axis significantly lowers accuracy of Earth observation and positioning.
Therefore, it is important to maintain a temperature of the honeycomb sandwich structure as uniform as possible through thermal control using a heater or the like so as to suppress the thermal deformation. It is also necessary to measure the temperature of the honeycomb sandwich structure on the orbit with high density and high accuracy so that precise thermal control is implemented.
Here, an optical fiber temperature sensor is given as one sensor configured to evaluate the temperature of a structure, such as the honeycomb sandwich structure. The optical fiber temperature sensor is a temperature sensor having the following features. The sensor has a small size and is lightweight, is strong against electromagnetic noise, and enables multipoint measurement. In addition, the optical fiber temperature sensor is, for example, a sensor using, as a sensor portion, a fiber Bragg grating (FBG) in which the Bragg wavelength of a reflectance spectrum changes with a temperature and a strain.
In general, in a system mounted with the optical fiber temperature sensor, a relationship between a Bragg wavelength and a temperature is actually measured in advance. A general optical fiber temperature sensor is configured to calculate a temperature corresponding to an acquired Bragg wavelength from the relationship between the Bragg wavelength and the temperature actually measured in advance.
An example of the configuration of the optical fiber temperature sensor is a configuration obtained by embedding a FBG sensor portion in a substrate made of carbon fiber reinforced plastic (CFRP) (see, for example, Patent Literature 1). The optical fiber temperature sensor described in Patent Literature 1 is configured to calculate a temperature from a relationship between a Bragg wavelength and the temperature.
In addition, another example of the configuration of the optical fiber temperature sensor is a configuration obtained by bonding a FBG sensor portion onto a bimetal (see, for example, Patent Literature 2). The optical fiber temperature sensor described in Patent Literature 2 is configured to calculate a temperature from a relationship between a strain caused in the bimetal by a temperature change and the temperature change through the utilization of a characteristic in which a Bragg wavelength changes with a strain.